千分位分隔符

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const thousandth = (num = 123456789, fixed = 0) => {     const strNum = num.toFixed(fixed)     const [startStr, endStr] = strNum.split('.')     let endIdx = startStr.length - 1     let n = 0     const res = []     while(endIdx > -1){         res.unshift(startStr[endIdx])         n++         if(n % 3 === 0 && endIdx !== 0){             res.unshift(',')         }         endIdx--     }     return endStr ? `${res.join('')}.${endStr}` : res.join('') }

　　从后往前 三位一切

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const thousandth = (num = 123456789, fixed = 2) => {     if (typeof num !== 'number') return new Error('请传入数字')     const [startStr, endStr] = num.toFixed(fixed).split('.')     const res = []     for (let i = startStr.length; i > -1; i -= 3) {         if (i > 3) {             res.unshift(',', ...startStr.slice(i - 3, i))         } else {             res.unshift(...startStr.slice(0, i))         }     }     return endStr ? `${res.join('')}.${endStr}` : res.join('') }